Lecture 2 Flashcards
1
Q
What do earthquakes result from? How is energy from an earthquake released? How are they mapped and measured?
A
- result from the rupture of rocks along a fault
- energy from an earthquake is released in the form of seismic waves
- mapped according to the epicentre, the focus is located directly below the epicentre
- they are measured by seismographs and compared by magnitude
2
Q
How can earthquake magnitude be measured quantitatively?
A
- expressed as a number to one decimal place; first developed by Richter but not used anymore
- Richter scale was a measure of strength of a wave at a distance of 100 km the epicentre
-Moment Magnitude Scale (M)
- scale is determined by area ruptured along a fault, amount of movement along the fault, elasticity of the crust at the focus
- similar to Richter scale, it is a logarithmic scale
- M7 earthquake represents 10x the amount of ground motion as an M6 earthquake
3
Q
What is the strongest earthquake to ever occur?
A
- M 9.5 in Chile in 1960
- In canada, it is M8.1 in BC in 1949
- except for very large earthquakes, Richter and M scale are similar
- there are only a few M9+ earthquakes each century
4
Q
How can earthquake intensity be measured qualitatively?
A
- Modified Mercalli Intensity Scale
- based on damage to structures and the effect on people
- based on 12 categories (1: felt by few people, 2: felt by a few people at rest especially on upper floors…12: damage is total and waves are seen on ground surface)
5
Q
Where are earthquakes most common?
A
- at or near plate boundaries
- motion at plate boundaries is not usually smooth or constant
- friction along plate boundaries exerts a force (stress) on the rocks, exerting strain or deformation
- when the stress exceeds the strength of the rocks, there is a sudden movement along a fault
6
Q
Describe the movement that occurs at a fault during an earthquake
A
- starts at the focus and propogates in all directions, called seismic waves
- faults are considered seismic sources
- identifying faults is necessary to evaluate risk of an earthquake in a given area
- not all faults reach the Earth’s surface
- blind faults are located below the surface
7
Q
How are earthquakes distributed geographically?
A
- they occur in the same places over and over again
- in Vancouver island, there is satellite monitoring for land that might be uplifting (when oceanic plate sinks it is crushing continental plate and may push it up a few mm)
- many in Oklahoma from fracking
8
Q
What are the two basic types of geologic faults?
A
- strike-slip faults: displacements are horizontal, San Andres (slabs move horizontally)
- dip-slip faults: displacements are vertical
9
Q
What are the 3 main types of dip-slip faults?
A
- reverse faults: hanging wall has moved up relative to footwall inclined at an angle steeper than 45 degrees
- thrust faults: these are similar to reverse faults except the angle is 45 degrees or less
- normal faults: the hanging wall has moved down relative to the footwall
- they are comprised of footwall and hanging wall
10
Q
How is fault activity determined?
A
- active: movement during the past 11600 years
- potentially active: movement during the past 2.6 million years
- inactive: no movement during the past 2.6 million years
11
Q
What is tectonic creep?
A
- the slow movement of rock or sediment along a fracture caused by stress
- also referred to as fault creep
- can damage roads and building foundations (movement of a few cm per decade)
- look for cracks in road, bricks of home, foundation chipping, driveway cracks
- along these faults periodic sudden displacements producing minor earthquakes can also occur
12
Q
What are different kinds of seismic waves?
A
- some travel within the body of the Earth and others travel along the surface
- body waves: P waves and S waves
- P waves: primary/compressional waves, move fast with a push pull motion and can travel through solids or liquids
- S waves: secondary/shear waves, move more slowly in an up and down motion and can only travel through solids
- surface waves: seismic waves that form when P and S waves reach Earth’s surface then move along it
- surface waves move more slowly than body waves
- responsible for damage near the epicentre
13
Q
What factors determine the shaking people experience during an earthquake?
A
- magnitude
- distance to the epicentre
- focal depth
- direction of rupture
- local soil and rock types
- local engineering and construction practices
14
Q
How does earthquake shaking appear on a seismograph?
A
- seismographs record the arrival of waves to a recording station
- because P waves travel faster than S waves, they appear first on a seismogram
- earthquake shaking decreases with distance from the epicentre
15
Q
How is distance to the epicentre determined?
A
- the difference between arrival times of the first P and S waves at different locations determine the distance to the epicentre
- the distance to the epicentre is calculated at 3 different seismic stations
- a circle with radius equal to that distance is drawn around the station
- epicetnre is located where the circles intersect; this is triangulation
16
Q
How does focal depth affect shaking at the surface?
A
- seismic waves become less intense as they spread outward toward the surface
- the greater the focal depth, the less intense the shaking at the surface
- this reduction of energy is referred to as attenuation
- waves spread the shaking out
17
Q
How does direction of rupture affect shaking?
A
- earthquake energy is focused in the direction of the rupture
- this is known as directivity and contributes to increased shaking
- radiated waves are sometimes stronger in one direction along the fault
18
Q
How does local soil and rock types affect shaking?
A
- dense homogenous crust can transmit earthquake energy quickly
- seismic energy slows down in areas with heterogeneous, folded, faulted crust
- implication: earthquakes in eastern NA are felt over larger areas than those in western NA
19
Q
What is amplification?
A
- an increase in ground motion during an earthquake
- P and S waves slow as they travel through alluvial sand, gravel, clay, soil
- alluvial: deposited by water, loose unconsolidated soil
- as the waves slow, some of their energy is transferred to surface waves
- has historically enhanced damage in San Fran area earthquakes
20
Q
What is the earthquake cycle?
A
- a hypothesis that explains successive earthquakes on a fault
- based on the idea that strain drops abruptly after an earthquake and then slowly accumulates until the next earthquake
- as stress continues to increase, the deformed material will eventually rupture
24
Q
Where are earthquakes located in Canada?
A
- africa was pushing into NA which created mountains
- in the north crust is rising which may account for earthquakes
32
Q
What is liquefaction?
A
- the transformation of water-saturated sediment from solid to liquid
- may occur during strong earthquakes when water pressure becomes high enough to suspend particles of sediment within the soil
- once the pressure decreases, the sediment compacts and regains its strength
- watery sands and slit may flow upward along fractures in the overlying solid material
- this effect can cause extensive damage
42
Q
How is seismic risk estimated?
A
- hazard maps identify areas of risk associated with earthquake effects
- they include areas prone to liquefaction, zones of possible ground rupture, historic epicentres
